(1) Field of the Invention
The present invention relates to a method for fabricating a semiconductor device and, more particularly, to a method for fabricating a thin-film capacitor in a semiconductor integrated circuit.
(2) Description of the Related Art
Conventionally, a thin-film capacitor formed in a semiconductor integrated circuit has a laminated structure of a silicon oxide (SiO.sub.2) and a silicon nitride (Si.sub.3 N.sub.4) with an electrode of polysilicon (also called polycrystalline silicon). In a dynamic random access memory (DRAM) device, a technique of making a capacitor portion after a transistor and a bit line are formed is disclosed in, e.g., "International Electron Devices Meeting Digest of Technical Papers, 1988", pp. 592-595.
The above conventional thin-film capacitor has a limit to the scaling down of the capacitor area in meeting the demand for a higher integration in an integrated circuit in recent years. It is, therefore, necessary to reduce the area of the capacitor portion effectively by means of realization of the thin-film and high permittivity and three-dimensional structure of the dielectric portion of the thin-film capacitor.
The conventional dielectric for forming a capacitor is an SiO.sub.2 or a Si.sub.3 N.sub.4. The permittivity or dielectric constant of these films is at most about 7, so that in order to obtain the required capacitance, a very small film-thickness of 10 nm or less is required in terms of the SiO.sub.2 film.
On the other hand, with such a thin film-thickness it is very difficult to realize a dielectric film which has a current-voltage characteristic lower than a tolerable leakage current. An electrode area may be effectively increased if use is made of a three-dimensional structure, but then there will be an increase in the leakage current due to reduction In the dielectric film thickness and concentration of the electric fields at a lower end portion of the electrode.
In order to obviate such a disadvantage, it has been proposed to adopt, as a material for a capacitor portion, SrTiO.sub.3 which has a dielectric constant approximately equal to 300 at room temperature, and a dielectric such as (Ba, Sr)TiO.sub.3, Pb(Zr, Ti)O.sub.3, Pb(Mg, Nb)O.sub.3 or Pb(Mg, W)O.sub.3 which has a higher dielectric constant, so that the required capacitance can be realized by using a thicker film than the SiO.sub.2 film. In this case also, however, an increase in the leakage current is inevitable owing to reduction in the dielectric film thickness and concentration of electric fields at the lower end portion of the electrode formed in a predetermined shape.